The present invention relates to non-steroidal mimetics of brassinolide and methods of their synthesis and use.
Brassinolide (22R, 23R, 24S)-2xcex1,3xcex1,22,23-tetrahydroxy-24-methyl-B-homo-7-oxa-5xcex1-cholestan-6-one, is a powerful plant growth-regulator.1 It manifests biological activity when applied exogenously at doses as low as one ng per individual plant to species such as rice, beans and others. Although brassinolide and related brassinosteroids such as castasterone are widespread throughout the plant kingdom, natural sources of brassinosteroids are an impractical source of these compounds because of their very low concentrations (typically ppb to ppt). While several syntheses of brassinolide and its analogues have been reported2, synthetic brassinosteroids are generally too expensive for most commercial applications. Despite the poor availability of brassinosteroids, a great deal of effort has been expended on investigations of their chemistry, biological properties, field applications, and molecular biology3-5. The discovery of alternative novel compounds capable of mimicking the biological activity of natural brassinosteroids would thus clearly be of considerable benefit if their synthesis were simpler and more cost-effective.
Numerous structure-activity studies of brassinosteroids have been reported3-13. In general, they reveal that the vicinal diol groups and the configurations of their stereocenters are of importance in maintaining high bioactivity. Certain methyl ether derivatives are also highly active14. The 5xcex1-configuration is required for optimum activity15, but the B-ring tolerates considerable variation, providing that the presence of a polar functional group, which does not have to be a lactone, is maintained16. Numerous side chain variations have also been shown to result in high bioactivity17.
The present invention comprises nonsteroidal brassinolide mimetics, methods of synthesizing such mimetics and methods of their use. Molecular modeling was used to determine the minimum energy conformation of brassinolide, which served as a starting point for the rational design of these nonsteroidal analogues. Modeling was followed by synthesis, using the procedures described herein. Potential mimetics were then bioassayed to determine their biological activity relative to brassinolide.
In one aspect, the invention comprises a non-steroidal mimetic of a brassinosteroid having two vicinal pairs of hydroxyl groups and a B-ring polar group, said mimetic comprising:
(a) two bicyclic subunits, wherein each subunit comprises a vicinal diol group in which the hydroxyl groups of each vicinal pair are cis and in a gauche relationship;
(b) a polar group attached to one bicyclic unit, said polar group corresponding to the B-ring polar group of a brassinosteroid; and
(c) a linker which joins the two bicyclic subunits such that each vicinal pair of hydroxyl groups and the polar group is substantially superimposed on the vicinal pairs and the B-ring lactone moiety of brassinolide respectively.
In one embodiment, the mimetic is a compound having the formula having the formula: 
wherein L represents an acetylene linker or a trans-ethylene linker, R represents a polar functional group, and Rxe2x80x2 represents hydrogen or hydroxy. In another embodiment, the mimetic is a compound having the formula: 
where L represents trans-ethylene.
The compounds of the present invention may also include those in which one or more of the hydroxyl groups are protected with removable protecting groups, such as hydrolyzable esters, ketals or acetals. As used herein, xe2x80x9cprotected hydroxylxe2x80x9d refers to a group which is readily converted to hydroxyl, for example, a hydrolyzable ester, a lower alkyl (i.e. C1 to C6), benzyl, trityl, allyl, or alkylsilyl ether, or an acetal (alkoxyalkyl ether). Since the hydroxyl groups of the subject compounds form cis-diols, cyclic acetals or ketals, e.g. acetonides, may also be used as protecting groups. Such protecting groups are widely used in organic synthesis and in preparation of prodrugs and are well known in the art. The protected hydroxyl form of the compound may itself have biological activity either per se or resulting from conversion or hydrolysis after application of the compound to the plant being treated.
In specific embodiments of the invention, the mimetics comprise four non-steroidal brassinolide analogue compounds, each with biological activity. These four compounds are:
(a) (xc2x1)-1,2-bis[4,6xcex1,7xcex1-trihydroxy-5,6,7,8-tetrahydronaphthyl]ethyne (mimetic 3) and its meso isomers;
(b) 1-[4,6xcex1,7xcex1-trihydroxy-5,6,7,8-tetrahydronaphthyl]-2-[6xcex1xe2x80x2,7xcex1xe2x80x2-dihydroxy-5xe2x80x2,6xe2x80x2,7xe2x80x2,8xe2x80x2-tetrahydronaphthyl]ethyne (mimetic 4) as two diastereomeric (xc2x1) pairs;
(c) (E)-(xc2x1)-1,2-bis[4,6xcex1,7xcex1-trihydroxy-5,6,7,8-tetrahydronaphthyl]ethene (mimetic 6) and its meso isomer; and
(d) (E)-(xc2x1)-1,2-bis[trans-(4axcex1,8axcex2-4-oxo-6xcex1,7xcex1-dihydroxy-4a,5,6,7,8,8a-hexahydro-(3H)-naphthyl]ethene (mimetic 11) and its meso isomer; or esters thereof.
In another aspect of the invention, there are provided methods for synthesizing non-steroidal mimetics of a brassinosteroid and methods and compositions for using such mimetics. In one aspect, the invention may comprise a method of promoting a desired tissue morphology and/or physiological state in a higher plant, wherein said desired tissue morphology or physiological state is selected from at least one of: shoot growth, grain, seed or fruit yield enhancement, root (radicle) growth retardation, improved fruit set and fruit quality or other desired tissue morphology or physiological state that is promoted by a brassinosteroid, said method comprising the step of applying an effective amount of a mimetic as claimed herein in a suitable delivery vehicle.